High-frequency oscillatory system



Oct. 25, 1932. E, R HENTSCHEL 1,884,679

HIGH FREQUENCY OSCILLATORY SYSTEM Filed Oct. 16, 1929 ATTORNEY Patented Oct. 25, 1932 UNITED STATES PATENT OFFICE ERNEST R. HENTSCHEL, OF WASHINGTON, DISTRICT OF COLUMBIA; JOHN OLSON, AD-

MINISTRATOR OF SAID ERNEST R HENTSCHEL, DECEASED, ASSIGNOR TO WIRED RADIO, INC., OF YORK, N. Y., A CORPORATION OF DELAWARE HIGH-FREQUENCY OSCILLATORY SYSTEM Application filed October 16, 1929.

My invention relates broadly to high frequency oscillating circuits, and more particularly to a constant frequency control system for oscillatory circuits.

One of the objects of my inventiontis to provide a high frequency oscillatory circuit in which high frequency oscillations are maintained at constant frequency by means of a piezo electric crystal element ground to a predetermined high frequency.

Another object of my invention is to provide an accurate system of frequency control in which oscillations of a predetermined frequency may be maintained constant under one condition of the circuit while self-oscillations in the circuit may be sustained under other conditions.

A further object of my invention is to provide an electron tube oscillatory system having circuits arranged forthe sustaining of self-oscillations or the maintenance of 0s cillations at a constant predetermined frequency under the control of a piezo electric crystal system by which oscillations corresponding to the frequency of the piezo electric crystal element are sustained while selfoscillations are prevented.

Other and further objects of my invention reside in the circuit arrangement for a high frequency oscillatory system, as more fully set forth in the specification hereinafter following and shown in the accompanying drawing which diagrammatically illustrates my invention.

My present invention is an improvement "i over the circuit arrangement shown in my application Serial No. 356,174, filed April 18, 1929, for signaling apparatus.

In my present invention I provide means whereby self-oscillations in an electron tube oscillatory system are prevented when the electron tube system is controlled by a piezo electric crystal operating at a predetermined high frequency. My invention is particularly adapted for the control of oscillatory circuits at frequencies lying within a range of LOGO-10,000 kilocycles. At frequencies around 5,000 kilocycles there is a tendency toward self-oscillation in the circuits of an electron tube oscillatory system. Such selfoscillations occur in a piezo electric crystal Serial No. 400,033.

controlled circuit by reason of the capacity efl'ects offered by the piezo electric crystal holder, where the fundamental frequency of the crystal tends to be masked by the ability of the associated circuits to sustain oscillations depending upon their own fundamental period, even though self-oscillations may not occur. Very thin crystals are employed to oscillate at frequencies as high as 5,000 kilocycles, and the condenser formed by the crystal electrodes and the dielectric consisting of the crystal acts as a very good by-pass condenser at frequencies such as 5,000 kilocycles. Feed back of considerable energy to the grid through the crystal can then occur and selfoscillations will be maintained. There is also a tendency for several frequencies to appear in the oscillatory circuits in addition to the desired frequencies brought out by the piezo electric crystal element. These other frequencies arise from the impression of excessive driving energy from the oscillatory circuits of the electron tube system.

My invention avoids the difficulties hereto fore experienced and enables oscillations in an electron tube system to be sustained either in accordance with the fundamental frequency of the piezoelectric crystal element or at frequencies due to oscillation of the oscillator circuits. I provide an electron tube system having input and output circuits, with means coupling the circuits for the sustaining of self-oscillations therein. In the input circuit I provide a switching device which may be opened or closed. In shunt with the switching system I connecta branch circuit including a piezo electric crystal element and an impedance in series. When the switching device is opened the branch circuit is effectively connected in the input circuit of the electron tube system. The series connected impedance is selected of such value with respect to the frequency of the piezo electric crystal element and the impressed electromotive force that sufiicient reactance is presented to prevent self-oscillations in the electron tube circuits while permitting the maintenance of oscillations at the frequency of the piezo electric crystal element.

The impedance may be a resistance, a condenser, or an inductance, but preferably an inductance is not employed because of the greater possibility of resonance effects which may fracture the crystal. The system consisting of thepiezo electric crystal and the condenser or other impedance connected in series offers a certain impedance atany given applied frequency. If for the.

frequencies to be applied the impedance of the system consisting of the crystal and the condenser or other impedance in series is small, enough energy may be fed back through the crystal to sustain self-oscillations. Since the frequency of self-"oscillations is determined by the tank circuit 5, 19, 20, which in operation is tuned approximately to the natural frequency of vibration of the crystal, the frequency of self-oscillations to be blocked by the auxiliary condenser or impedance is approximately the same as the crystal frequency. The capacity of the auxiliary series condenser or other impedance, should therefore be so chosen that with the condenser consisting of the, piezo electric crystal and its electrodes, only a comparatively small amount of energy will be fed back throughthe crystal to the grid at frequencies near the crystal frequency. The amount of energywhich must reach the grid to sustain self-oscillations dependson the kind oftube employed, the values of inductance and capacity employed, particularly in the tank circuit, and the frequency.

Referring to the drawing inmore detail, reference character 1 designates an electron tube including cathode 1a control electrode 1?) and plate 10. The cathode 1a is by-passed by condenser 2 and'the cathode supplied with energy from any suitable source, such as battery 3 under control of rheostat4. The plate circuit includes electrode 10, inductance '5'. meter 6, telephones? and suitable source of high frequency potential 8, returningto the cathode electrode 1a. The input circuit includes grid electrode 16, condenser 9,.switching device'15 and inductance 17, returning to the cathode 1a. A leak path is provided as shown at 18 between grid electrode 16 and cathode 1a. The input and output circuits may be electrostatically coupled through condenser 19and variable condenser 20, or there may be suitable inductive coupling between inductances '5 l and 17. The value, of inductance 17 is so chosen that self-oscillations occur when switch 15 is closed. After inductance 17 is chosen the value of condenser impedance element 14 may be chosen'as has been outlined.

I have shown for purposes of explaining my invention a connection to the input circuit from an antenna 21 through series condenser22to one side of the input circuit and a connection to ground 23 from the other side of the input circuit, but it will be understood that any source ofimpressed radio frequency branch circuit 10 connected in shunt with switching device 15. The branch circuit 10 includes piezo electric crystal 12 which is supported in a suitable holder having electrodes 12a and 12b establishing connection to .theoppositefaces of the piezo electric crystal element. In series vwith the piezo electric crystal element, I provide an impedance 14. For purposes of explanation I have illustrated the impedance 14 as a condenser, but I desire that it be understood that either an inductance or impedance of proper value may be employed. The switching device 15 is automatically opened when the piezo electric crystal element 12 and the series connected condenser-14 are interposed in the in put circuit. Various instrumentalities may be employed for effecting this switching operation automatically. I have represented by dotted lines 16 the cooperation of switching device 15 with the interposition of piezo electric crystal element 12 in the circuit. Reference to my copending application hereinbefore referred to may be made for a more detailed disclosure of the constructionof one form of crystal mounting and associated switching devices which may be employed.

The impedance 14 is so selected that oscillations will not be sustained in the electron tube circuits when switch 15 is opened unless the piezoelectric crystal 12'is generating an e'lectromotive force. The inductance 17 may be regarded as the source of radio frequency energy from which current flows in its passage through the impedance 14 and through piezo electric crystal 12. The inductance 17 and impedance 14 are proportioned so that sufficient current is forced through impedance 14 at the frequency of the piezo electric crystal element to sustain oscillations in the electron tube-circuits when the piezo electric crystal element is oscillating. Under these conditions self-oscillation in the electron tube circuits is prevented. However, by closing switch 15 the piezoelectric crystal element system is rendered ineflective and self-oscillations in the electron tube circuits may be established.

The circuit arrangement of my invention has-proven to be very effective in oscillatory circuits for frequency meters adapted for measurement of high frequency of the order hereinbefore stated. The advantages of providingeither a self-oscillating or crystal controlled frequency meter will be understood when practical operating conditions are considered in which instances may arise, wherein the piezo electric crystal element may cease oscillating, in which event switching device 15 may be closed and the use of the freqeuncy meter continued without interruption.

While I have described my invention in one of its preferred embodiments, I desire that it be understood that modifications may be made and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. In a high frequency oscillatory system,

an electron tube having cathode, grid and anode, a piezo electric crystal element, an 1nput circuit connected between said grid and said cathode and comprising said piezo electric crystal element, an output circuit connected between sald anode and said cathode,

and means connected 1n series with said plezo electric crystal element for preventing selfoscillations when said crystal is not vibrating, said means comprising an impedance of admittance substantially less than is required to pass through the circuit containing said piezo electric crystal element and said impedance at frequencies approximating the frequency of said crystal element suflicient current to maintain self-oscillations when said crystal is not vibrating.

2. In a high frequency oscillatory system. an electron tube having cathode, grid and anode a piezo electric crystal element, an input circuit connected between said grid and said cathode and comprising said piezoelectric crystal element, an output circuit connected between said anode and said cathode, and means connected in series with said piece .10 electric crystal element for preventing selfosoillations when said crystal is not vibrating, said means comprising a condenser of capacity substantially less than is required to pass through the circuit containing said piezo 5 electric crystal element and said condenser at frequencies approximating the frequency of said crystal sui'ncient current to maintain self-oscillations when said crystal is not vibrating.

5 3. In a crystal controlled thermionic os cillator having a piezo electric crystal in the input circuit, the method of preventing selfoscillations due to back coupling between the input and output circuits which consists in inserting in series with the piezo electric crystal in the input circuit, a condenser having a capacty substantially less than the value of capacity so connected in series for which the thermionic oscillator and connected circuits with said crystal connected in the input circuit will produce self-oscillations when said crystal is not vibrating.

ERNEST R. HENTSCHEL. 

